Crystalline and magnetic structures, magnetization, heat capacity, and anisotropic magnetostriction effect in a yttrium-chromium oxide

We have studied a nearly stoichiometric insulating ${\mathrm{Y}}_{0.97\left(2\right)}{\mathrm{Cr}}_{0.98\left(2\right)}{\mathrm{O}}_{3.00\left(2\right)}$ single crystal by performing measurements of magnetization, heat capacity, and neutron diffraction. Albeit that the $\mathrm{Y}\mathrm{Cr}{\mathrm{O}}_3$ compound behaves like a soft ferromagnet with a coersive force of $\sim 0.05$ T, there exist strong antiferromagnetic (AFM) interactions between ${\mathrm{Cr}}^{3+}$ spins due to a strongly negative paramagnetic Curie-Weiss temperature, i.e., $-433.2$(6) K. The coexistence of ferromagnetism and antiferromagnetism may indicate a canted AFM structure. The AFM phase transition occurs at $T_{\text{N}}=141.5\left(1\right)\mathrm{K}$, which increases to $T_{\text{N}}$(5 T) = 144.5(1) K at 5 T. Within the accuracy of the present neutron-diffraction studies, we determined a G-type AFM structure with a propagation vector k = (1 1 0) and ${\mathrm{Cr}}^{3+}$ spin directions along the crystallographic $c$ axis of the orthorhombic structure with space group $Pnma$ below $T_{\text{N}}$. At 12 K, the refined moment size is 2.45(6) ${\mu }_{\text{B}}, \sim 82\%$ of the theoretical saturation value $3{\mu }_{\text{B}}$. The ${\mathrm{Cr}}^{3+}$ spin interactions are probably two-dimensional Ising like within the reciprocal (1 1 0) scattering plane. Below $T_{\text{N}}$, the lattice configuration ($a$, $b$ , $c$ , and $V$ ) deviates largely downward from the Grüneisen law, displaying an anisotropic magnetostriction effect and a magnetoelastic effect. Especially, the sample contraction upon cooling is enhanced below the AFM transition temperature. There is evidence to suggest that the actual crystalline symmetry of $\mathrm{Y}\mathrm{Cr}{\mathrm{O}}_3$ compound is probably lower than the currently assumed one. Additionally, we compared the $t_{2\text{g}}\mathrm{Y}\mathrm{Cr}{\mathrm{O}}_3$ and the $e_{\text{g}}{\mathrm{La}}_{7/8}{\mathrm{Sr}}_{1/8}{\mathrm{MnO}}_3$ single crystals for a further understanding of the reason for the possible symmetry lowering.

Figure 1

Orthorhombic crystal structure (with space group $Pnma$) with one unit cell (solid lines) and the AFM structure in one AFM unit cell with the propagation vector at k = (1 1 0) below $T_{\text{N}}=141.5\left(1\right)$ K of the $\mathrm{Y}\mathrm{Cr}{\mathrm{O}}_3$ single crystal. The arrows on the Cr ions represent the spins of chromium. Both the unit cells of orthorhombic and AFM structures are ($a b c$).

Figure 2

(a) ZFC and FC magnetization ($M$) of chromium ions in the single-crystal $\mathrm{Y}\mathrm{Cr}{\mathrm{O}}_3$ compound as a function of temperature measured at ${\mu }_{0}H=0.01$ T. (b) Corresponding ZFC and FC inverse magnetic susceptibility ${\chi }^{-1}$ (circles) of chromium ions in the single-crystal $\mathrm{Y}\mathrm{Cr}{\mathrm{O}}_3$ compound versus temperature. The dash-dotted line indicates a CW behavior of the ZFC data at elevated temperatures between 200 and 300 K, which was extrapolated to ${\chi }^{-1}=0$ to show the PM Curie temperature ${\theta }_{\text{CW}}$. The fit results were listed in Table 1. In (a) and (b), $T_{\text{N}}=141.5$(1) K labels the AFM transition temperature at ${\mu }_{0}H=0.01$ T, and the solid lines are guides to the eye.

Figure 3

ZFC magnetic hysteresis loop of the single-crystal $\mathrm{Y}\mathrm{Cr}{\mathrm{O}}_3$ compound measured at 2 K. Inset (a) is the enlarged image of the narrow loop.

Figure 4

Heat capacities of the single-crystal $\mathrm{Y}\mathrm{Cr}{\mathrm{O}}_3$ compound measured at 0 T (solid circles) and 5 T (void circles). The solid lines are guides to the eye. Inset (a) is the enlarged image around the AFM transition temperatures. The vertical dashed lines show the detailed transition temperatures at the fields of 0 and 5 T. Here, $T_{\text{N}}$(0 T) = 141.5(1) K at 0 T; by comparison, at 5 T, $T_{\text{N}}$(5 T) = 144.5(1) K. The solid lines are guides to the eye.

Figure 5

Observed (circles) and calculated (solid lines) time-of-flight neutron-powder diffraction (NPD) patterns of a pulverized $\mathrm{Y}\mathrm{Cr}{\mathrm{O}}_3$ single crystal, collected from the POWGEN diffractometer (SNS, USA) at 12, 145, and 300 K. The vertical bars mark the positions of nuclear (up, space group $Pnma$) and magnetic (down, space group $P$-1) Bragg reflections, and the lower curves represent the difference between observed and calculated patterns.

Figure 6

Refined chromium-moment size $M_{\text{ref}}$ (at 0 T) of a pulverized $\mathrm{Y}\mathrm{Cr}{\mathrm{O}}_3$ single crystal versus temperature by the software of FULLPROF SUITE [44]. The solid line is a guide to the eye. Error bars are standard deviations obtained from our FULLPROF refinements in the $Pnma$ symmetry. $T_{\text{N}}=141.5\left(1\right)$ K labels the AFM transition temperature at zero applied-magnetic field.

Figure 7

(a) Representative longitudinal scans of the magnetic Bragg (1 1 0) reflection at three temperatures of 2, 195, and 300 K, from the D23 (ILL, France) study on a $\mathrm{Y}\mathrm{Cr}{\mathrm{O}}_3$ single crystal. The solid lines are guides to the eye. (b) Corresponding temperature-dependent integrated intensities of the magnetic Bragg (1 1 0) reflection. $T_{\text{N}}=141.5$(1) K labels the AFM transition temperature. The solid line was a fit to Eqs. (2) and (4) in the affiliated thermal regime. It was extrapolated to overall temperatures (dash-dotted line). The error bars in (a) and (b) are the standard deviations based on our measurements and fits.

Figure 8

Subtracted integrated intensity from the pure magnetic contribution at the Bragg (1 1 0) peak position (void pentagons), to see detailed analysis in the text. Inset (a) shows the enlarged image around $T_{\text{N}}$ from 132–145 K. The solid line was a fit to the power-law Eq. (5) in the affiliated thermal regime. The error bars are the propagated standard deviations based on our calculations.

Figure 9

(a) Temperature-dependent lattice constants $a, b$, and $c$ of a pulverized $\mathrm{Y}\mathrm{Cr}{\mathrm{O}}_3$ single crystal. (b) Corresponding anomalous unit-cell volume $V$ expansion with temperature. The solid lines in (a) and (b) are theoretical estimates of the variation of structural parameters using the Grüneisen model with Debye temperature of ${\theta }_{\text{D}}=580$ K that is the same value as the one reported previously [14]. $T_{\text{N}}=141.5\left(1\right)$ K labels the AFM transition temperature. The error bars in (a) and (b) are the standard deviations obtained from our FULLPROF refinements with the $Pnma$ structural symmetry.

Figure 10

(a) Three bond lengths of Mn-O1, Mn-O21, and Mn-O22 in the ${\mathrm{La}}_{\frac{7}{8}}{\mathrm{Sr}}_{\frac{1}{8}}{\mathrm{MnO}}_3$ single crystal [40] versus temperature. $T_{\text{JT}}\approx 180–270\mathrm{K}$ denotes the regime of the Jahn-Teller effect. (b) Corresponding bond lengths in the $\mathrm{Y}\mathrm{Cr}{\mathrm{O}}_3$ single crystal as a function of temperature from this study. $T_{\text{N}}=141.5$(1) K labels the AFM transition temperature. The error bars in (a) and (b) were from our FULLPROF refinements.

Figure 11

Schematic illustration of the three Cr-O bonds (Cr-O1, Cr-O21, and Cr-O22), as well as the two bond angles Cr-O-Cr (Cr-O1-Cr and Cr-O2-Cr) in the orthorhombic structure of a $\mathrm{Y}\mathrm{Cr}{\mathrm{O}}_3$ single crystal. In this structural symmetry (with space group $Pnma$), Cr ions in $\mathrm{Y}\mathrm{Cr}{\mathrm{O}}_3$ compound have the same Wyckoff site, $4b$ (0 0 0.5), as that of the Mn ions in ${\mathrm{La}}_{\frac{7}{8}}{\mathrm{Sr}}_{\frac{1}{8}}{\mathrm{MnO}}_3$ compound [13, 40, 56, 57].

Figure 12

(a) Temperature-dependent bond angles of Mn-O1-Mn and Mn-O2-Mn in the ${\mathrm{La}}_{\frac{7}{8}}{\mathrm{Sr}}_{\frac{1}{8}}{\mathrm{MnO}}_3$ single crystal [40]. $T_{\text{JT}}\approx 180–270\mathrm{K}$ denotes the regime of the Jahn-Teller effect. (b) Temperature-dependent bond angles of Cr-O1-Cr and Cr-O2-Cr in the $\mathrm{Y}\mathrm{Cr}{\mathrm{O}}_3$ single crystal from the present study. $T_{\text{N}}=141.5$(1) K labels the AFM transition temperature. The error bars in (a) and (b) are the standard deviations from refinements. The solid lines in (a) and (b) are guides to the eye.

Figure 13

Comparison of the averaged bond lengths of Y-O in $\mathrm{Y}\mathrm{Cr}{\mathrm{O}}_3$ (left, from the present study) and La-O in ${\mathrm{La}}_{\frac{7}{8}}{\mathrm{Sr}}_{\frac{1}{8}}{\mathrm{MnO}}_3$ [40] (right) single crystals. $T_{\text{JT}}\approx 180–270\mathrm{K}$ denotes the regime of the Jahn-Teller effect of ${\mathrm{La}}_{\frac{7}{8}}{\mathrm{Sr}}_{\frac{1}{8}}{\mathrm{MnO}}_3$ compound. $T_{\text{N}}=141.5$(1) K labels the AFM transition temperature of $\mathrm{Y}\mathrm{Cr}{\mathrm{O}}_3$ compound. The error bars are the calculated standard deviations. The solid lines are guides to the eye. It is clear that the bond length of $\langle \mathrm{Y}-\mathrm{O}\rangle$ is shorter than that of the $\langle \mathrm{La}-\mathrm{O}\rangle$ bond beyond statistics.